In the U.S. alone there are approximately 50,000 tons of semi-dry sewage sludge produced daily in the U.S. and the treatment, disposal and use of this material presents a major problem to this country on national, state and local levels.
For the most part, sewage sludge has been disposed of after treatment, as opposed to any reformation process producing commercially viable by-products. This disposal includes incineration, landfill, lagooning, stockpiling, ocean-disposition and land-spread. Disposal, however, while commonly practiced, is quite expensive and environmentally dangerous in many applications.
Incineration is generally found uneconomical because of the high cost of incineration and ash disposal equipment and the relatively low net heat yield. Incineration to be productive requires pre-drying the sludge but the equipment costs are exhorbitantly high and the gaseous discharges are unacceptable for most community environments.
There have been a wide variety of purely experimental systems developed for the pyrolysis of sewage sludge, but they have been without exception economically unjustifiable as a method of disposing of the sewage by-products.
One such project involved the experimental conversion of sewage sludge to oil by hydroliquefaction at the Worcester Polytechnic Institute in 1983. These experiments, however, are not acknowledged by applicant to be prior art since applicant's system and process were reduced to practice prior to this work conducted by Wilmer L. Kranich at Worcester Polytechnic Institute, and his experiments are being noted in this background solely for the purpose of acknowledging contemporaneous activity in hydrogenation of sewage sludge and the problems noted in this contemporaneous activity.
In 1983 Mr. Kranich performed hydroliquefaction experiments with sewage sludge using two methods (1) an in situ (batch) hydrogenation on as-received aqueous suspensions of sewage sludge, and (2) the hydrogenation of dried samples in an oil carrier in situ (also a batch experiment). The only special preparation of the as-received raw materials was to stir the raw contents of a sample to obtain an as representative sample as possible. The dried samples were prepared in two ways; the first involved direct low temperature (below 100 degrees C.) drying in a nitrogen atmosphere to avoid oxygenation by air. In the second method the sludge was first de-watered in the laboratory centrifuge and the supernatant discarded before drying. The difference in the results caused by the presence of the dried dissolved salts was not measurable within the limits of accuracy of the experiment. The dried material was ground to a fine powder and slurried in carrier oil in the second method.
In the oil carrier runs, a slurry of dried powdered sludge was dispersed in an anthracine or paraffin oil in a small beaker, warmed to a free-flowing mixture, and with an added catalyst was placed in an autoclave. Hydrogen from an external source under pressure was added to the autoclave until the initial reaction pressure was reached.
This dried sewage sludge was pyrolized at temperatures between 410 to 440 degrees C. with reaction times of 10 to 20 minutes in oil slurries.
The oil and gas content of the yields from Mr. Kranich's hydrogenation process was inconsistent and inclusive, because the amount of carrier oil lost in his calculations by conversion to gas resulted in a calculated yield greater than the total amount of dry sludge converted, acknowledged by Mr. Kranich to be "an impossible result".
As an alternative to immersing the sewage sludge in an oil carrier, Mr. Kranich also attempted batch pyrolysis with sewage sludge using a water-sewage sludge slurry without the addition of any carrier oil. He hypothesized that the pyrolysis of this slurry would produce hydrogen that ideally may produce hexane C.sub.6 H.sub.14 but his experiments were a total failure in that no such results occurred. Again the entire hypothesis of Mr. Kranich's experiment was the addition of externally stored hydrogen.
Mr. Kranich concluded, based upon his experiments, that the conversion of dried sludge slurried in oil was not economically feasible in view of the large equipment and operating costs for pre-drying the sludge and therefore his subsequent experiments were directed to batch testing a water slurried sewage sludge without any oil carrier whatsoever. The results of these latter experiments did not indicate any significant hydrogenation because of the low temperature limitation, i.e. around 300 degrees C., imposed by the large amounts of carrier water.
It is a primary object of the present invention to ameliorate the problems noted above in the conversion of sewage sludge to oil. Also, the invention may be used to process a large variety of other organic waste products to generate oils and burnable gases.